There are no licensed vaccines for human cytomegalovirus (HCMV). Limited clinical trials have been conducted with live attenuated and recombinant subunit vaccines. Both vaccine strategies induced host immune responses similar to those observed following HCMV infection. Despite partial success protecting from disease in renal transplant recipients, the goal of developing an HCMV vaccine that elicits protective immunity has not been achieved. There are critical impediments to the development of an HCMV vaccine. Design of an effective HCMV vaccine requires (i) characterization of the correlates of protective immunity and (ii) a better understanding of HCMV natural history. Both aspects of HCMV are incompletely resolved and difficult to investigate rigorously in human populations. Several studies have suggested that two measures of humoral immunity, neutralizing antibodies and antibody avidity, and one measure of cellular immunity, CTL, are useful for assessing protective anti-HCMV immunity. The two HCMV proteins associated with protective immune responses, gB and pp65, represent starting points for any rational vaccine. Recent data on HCMV and the closely related rhesus CMV (RhCMV) strongly implicate viral modulation of host immune responses as a critical component of CMV natural history. CMV appears to have evolved strategies that alter lymphoid cell signaling and trafficking. These include ORF with homologies to cellular proteins, such as interleukin- 10 (IL-10), alpha-chemokines, and beta-chemokine receptors. There are limited functional data for these proteins in vitro. Virological roles in vivo remain speculative. Based on sequence homologies, it is reasonable to infer that HCMV has targeted pro-inflammatory immune responses for disruption during infection. Attenuation of HCMV's ability to modulate host immune responses should limit viral replication and disease sequelae. Accordingly, HCMV vaccines must be directed against both structural and immune modulating ORF to reduce virologic parameters of infection and/or disease. In other words, protective immunity will be enhanced when vaccination is directed against identified immunogens, such as gB and pp65, together with novel vaccine targets represented by immune modulating ORF. This proposal will use the rhesus macaque model of HCMV to test whether RhCMV encoded IL-10 and beta-chemokine receptors are critical elements involved in the corruption of host immune responses through the following Aims: (1) Construction of RhCMV variants with deletion of either IL-10 or beta-chemokine receptors. (2) In vivo analysis of RhCMVdeltaIL-10 and RhCMVdeltaUS28. (3) Immunological determinants of RhCMV shedding frequency. This proposal is designed to determine whether disruption of virus modulation of host immune responses impairs CMV replication and dissemination to distal sites of persistence. A successful outcome of this approach will demonstrate that attenuation of the CMV immunomodulatory ORF by immunization represents a rational vaccine strategy. This would fundamentally alter the paradigm for vaccine approaches to HCMV.